1. Field of the Invention
The present invention relates to multilayered polymeric films and processes for making the same. More particularly, the invention pertains to polymethylpentene containing films having improved mechanical and release properties when exposed to high temperatures.
2. Description of the Prior Art
Fluorinated ethylene propylene films are commercially used as release films in producing aerospace composite structural elements under nitrogen or air pressurized heated autoclave conditions. Materials for such structural elements may contain fiber reinforced epoxy or phenolic resins. However, such films are very expensive, have a low tear strength, transfer fluorinated contaminants to the finished composite surface, is difficult to color, has a specific gravity of greater than 2.1 g/cc, and cannot be readily incinerated due to it's fluorine content.
Polymethylpentene (PMP) films such as poly-4-methyl-1-pentene film are also commercially used as release films in producing aerospace composite structural elements under nitrogen or air pressurized heated autoclave conditions. Materials for such structural elements may contain fiber reinforced epoxy or polyacrylate resins. However, PMP film is relatively expensive, have very low tear strength, high stiffness or modulus which limits its use to the production of flat parts, cannot be produced at thicknesses of less than 1.5 mil (38.1 microns) due to low tensile strength and has an upper use temperature of 177.degree. C.
It is known in the art to produce multilayered polymeric films containing a polymethylpentene ("PMP") layer and a thermoplastic layer. For example, U.S. Pat. No. 5,106,692 discloses a delamination resistant structure of layers of PMP and various thermoplastic resins in which a specific adhesive interlayer blend is sandwiched between the layers. Thermoplastic resins may be polyamides. U.S. Pat. No. 5,858,550, which is incorporated herein by reference describes a biaxially oriented, non-thermally stabilized five layer coextruded film which has a PMP/tackifier/polyamide/tackifier/PMP construction. This film would not be acceptable as a very high temperature release film for aerospace composite structural elements because the biaxial orientation would result in release film shrinkage at elevated temperatures, resulting in the formation of defects in the surface of the composite being fabricated. In addition, the orientation process significantly increases the modulus of the film which would prohibit its use in the fabrication of complex or curvilinear composites. U.S. Pat. No. 5,080,979 describes a non-thermally stabilized multilayer laminate contain PMP/tackifier/polyamide for use as a release film in the fabrication of either rigid or flexible printed circuit boards. This film would not be acceptable for aerospace composite structural element manufacture because it would have insufficient thermal stability to withstand air autoclave pressurization, and would tend to curl, producing surface blemishes on the finished composite structure. U.S. Pat. No. 5,106,692 describes a non-thermally stabilized three layer coextruded film having the structure PMP/tackifier/polyamide. This film would not be acceptable for aerospace composite structural elements because it would have insufficient thermal stability to withstand air autoclave pressurization, and would tend to curl, producing surface blemishes on the finished composite structure.
PMP films are desirable due to their good water barrier properties as described in U.S. Pat. No. 5,079,052 and for release films for high temperature composites as described in U.S. Pat. No. 5,080,979. However, because such films typically lose their dimensional stability at temperatures in excess of about 350.degree. F. (177.degree. C.), these films must be cured at lower temperatures for a long period of time. It would be desirable to produce a PMP-containing film that retains its dimensional stability and release properties at high temperatures. It would further be desirable to reduce the production cycle time for high temperature composites by using a release film which can be cured both rapidly and without leaving a residue on the surface of the composites.
The present invention produces a thermally stabilized multilayered PMP/polyamide containing release film which overcomes the above problems associated with release films used in either air or nitrogen autoclaves at temperatures of up to 210.degree. C. This significant increase in upper temperature is unexpected considering the crystalline melting points the materials used in the release film. In addition, the thermal stability of the film is maintained after exposure to air at high temperatures and pressures for extended periods of time. It has the drapability or flexibility equal to that of fluorinated ethylene propylene film. These coextruded films also significantly reduce manufacturing costs to the composite manufacturer. The film has acceptable surface quality and interlayer bond strength to produce defect free surfaces on cured structural composites.
In addition, the significant viscosity mismatch between the polymer melts employed to produce the film theoretically suggests that the viscosity of the nylon core should be reduced in order to produce an acceptable film, but it has been discovered that the opposite is true. In addition, one skilled in the art would normally expect to increase casting roll temperatures in order to increase interlayer bond strength of the crystallized coextrusion. It has also been discovered that the opposite is true for this film.